1. Field of the Invention
The present invention relates generally to a metal hydride negative electrode for use in a nickel hydride, and a method for producing the same. More specifically, the invention relates to a metal hydride negative electrode having excellent discharge characteristics at the beginning of a charge and discharge cycle, an excellent gas absorptivity during charge, and excellent cycle life initial characteristics, and a method for producing the same.
2. Description of the Prior Art
In recent years, many secondary cell capable of being repeatedly charged are used. A nickel hydride cell serving as a typical secondary cell comprises an alkaline electrolyte filled in a cell vessel, a metal hydride negative electrode containing a hydrogen absorbing alloy as a principal component, a positive electrode containing a metallic oxide as a principal component, and a separator provided between the metal hydride negative electrode and the positive electrode.
As characteristics required for nickel hydride cells, there are excellent discharge characteristics at the beginning of a charge and discharge cycle, an excellent gas absorptivity during charge, and an excellent cycle life.
In order to improve these characteristics, many proposals are made. For example, Japanese Patent Laid-Open No. 6-290775 discloses a method for producing a metal hydride electrode, which has improved initial discharge characteristics, an improved gas absorptivity during charge and an improved cycle life, by immersing hydrogen absorbing alloy powder in an alkaline solution to reform the surface thereof and carrying out an alkaline treatment at a higher temperature again while a metal hydride negative electrode is formed.
However, there is a problem in that it takes a lot of time to produce the metal hydride negative electrode described in the publication since it is required to repeatedly carry out alkaline treatments.
For that reason, it is desired to produce a metal hydride electrode having excellent initial discharge characteristics, gas absorptivity during charge and an excellent cycle life without the need of any complicated producing processes.
It is therefore an object of the present invention to eliminate the aforementioned problems and to provide a metal hydride negative electrode having excellent discharge characteristics at the beginning of a charge and discharge cycle, excellent gas absorptivity during charge, and an excellent cycle life, and a method for producing the same without the need of any complicated producing processes.
In order to accomplish the aforementioned and other objects, after the inventor has diligently studied, the inventor""s attention is turned to the fact that the surface portion of a negative electrode plate is formed so as to have a predetermined water repellent rate and a plurality of convex portions, so that the inventor has made the present invention on the basis of this knowledge.
That is, in order to accomplish the aforementioned and other objects, according to one aspect of the present invention, there is provided a metal hydride negative electrode for use in a nickel hydride cell, the metal hydride negative electrode comprising a substrate and a negative electrode plate which is formed by applying a hydrogen absorbing alloy composition containing a hydrogen absorbing alloy powder, a conductive material, a binder and a dispersing agent on the substrate, wherein the negative electrode plate has a surface portion which has a predetermined water repellent rate and a plurality of convex portions.
The surface portion may absorb a volume expansion portion, which is produced by repeating charge and discharge cycles, to cause the convex portions to disappear to be flattened.
Preferably, the predetermined water repellent rate exceeds 40%.
The convex portions may be formed on the basis of the difference in specific gravity between the hydrogen absorbing alloy powder, the conductive material, the binder and the dispersing agent in the hydrogen absorbing alloy composition.
Preferably, the conductive material is carbon powder, the binder is a polytetrafluoroethylene, the dispersing agent is water.
The convex portions may mainly comprise the conductive material and the binder.
Preferably, the convex portions have a height of 15 to 55 xcexcm, and the distance between adjacent two of the convex portions is in the range of from 1.3 mm to 4.8 mm.
According to another aspect of the present invention, there is provided a method for producing a metal hydride negative electrode for use in a nickel hydride cell, the method comprising the steps of: arranging a substrate on an application table; producing a paste-like hydrogen absorbing alloy composition containing a hydrogen absorbing alloy powder, a conductive material, a binder and a dispersing agent; and applying the hydrogen absorbing alloy composition on the substrate to dry and press the hydrogen absorbing alloy composition to form a negative electrode plate, wherein at the step of producing the hydrogen absorbing alloy composition, the amount of the dispersing agent with respect to the amount of the hydrogen absorbing alloy powder is set to be a predetermined part by weight so that the negative electrode plate has a surface portion having a water repellent property and a plurality of convex portion.
Preferably, the dispersing agent is water, and the amount of the dispersing agent is in the range of from 30 to 60 parts by weight with respect to 100 parts of the hydrogen absorbing alloy powder by weight.
At the step of arranging the substrate on the application table, the substrate may be horizontally arranged on the application table.
As described above, according to the present invention, the surface portion of the negative electrode plate has a predetermined water repellent rate and a plurality of convex portions. If the surface portion of the negative electrode thus has the predetermined water repellent rate and the plurality of convex portions, it is possible to provide a metal hydride negative electrode without the need of any complicated producing processes, by utilizing the difference in specific gravity between components of the hydrogen absorbing alloy composition and by setting the amount of the dispersing agent to be a predetermined part by weight with respect to the hydrogen absorbing alloy powder.
In the final stage of charge and during overcharge, oxygen gas is produced. This oxygen gas is dispersed toward the metal hydride negative electrode via the separator. In this case, if the surface portion of the metal hydride negative electrode has a high water repellent property, the surface portion is easy to absorb oxygen gas, so that the oxygen gas trapped into the surface portion recombines with hydrogen atoms which are absorbed into the metal hydride negative electrode, thereby inhibiting the internal pressure rise in the cell vessel.
If charge is started, the charging voltage rises with the elapse of time. In the final stage of charge, the charging voltage reaches the maximum charging voltage, so that the charging voltage rise is stopped. When oxygen gas effectively recombines with hydrogen atoms, heat is generated to raise the temperature in the cell vessel, so that there is caused a phenomenon that the charging voltage drops immediately after the charging voltage reaches the maximum charging voltage. By utilizing this phenomenon, it is possible to very surely and easily detect the charging time until the charging voltage reaches the maximum charging voltage. Therefore, by effectively causing the recombination of oxygen gas with hydrogen atoms in the metal hydride alloy electrode, it is possible to surely detect the maximum charging voltage to timely control the termination of charge, so that it is possible to surely avoid overcharge and increase the cycle life.
When charge and discharge cycles are repeated, a large number of hydrogen atoms, which do not contribute to discharge, accumulate in the hydrogen absorbing alloy. The plurality of fine convex portions existing on the surface portion of the metal hydride negative electrode absorbs the volume expansion to prevent the thickness of the electrode and to inhibit the separator from being pressed and crushed in the cell vessel in a direction perpendicular to the separator, so that it is possible to prevent the decrease of the amount of the electrolyte arranged between the positive electrode and the metal hydride negative electrode, i.e., the amount of the electrolyte contained in the separator. As a result, it is possible to inhibit the lowering of the discharging voltage, so that it is possible to increase the cycle life.